Conventional electret condenser microphones (ECMs) are widely available and used in significant volumes in numerous consumer products including toys, hearing aids, and cell phones. Replacing the traditional ECM with batch processed silicon microphones is based on meeting or exceeding the performance and cost of the ECM in high volume. The cost of a silicon microphone is proportional to the product of its complexity, i.e. number of mask steps, and its size. In order to scale down a microphone to very small size, a number of different design and process issues must be mastered.
U.S. Pat. No. 5,408,731 to Berggvist et al. shows one way of making a silicon microphone. Berggvist et al. discloses a single crystal silicon diaphragm rigidly supported at its edges by a silicon frame etched from the handle wafer. The minimum size of this device is based on the diaphragm size needed to achieve the desired sensitivity plus the amount of frame area needed to properly support the diaphragm. Fully clamped diaphragms are very stiff for their size. In addition, the process requires forming a connecting layer, and after etching the first substrate to form the diaphragm, the process requires the step of eliminating a part of the connecting layer which is located between the diaphragm and the part of the second substrate to form an open space between the diaphragm and the second substrate. The present invention alleviates the need for forming a connecting layer and eliminating a part of this connecting layer which is located between the diaphragm and the part of the second substrate to form an open space between the diaphragm and the second substrate, as will become apparent from the description below.
U.S. Pat. No. 5,490,220 to Loeppert discloses that simply supported diaphragms are more compliant and can be made smaller to achieve the same performance.
The capacitance between the flexible diaphragm and the rigid backplate of a capacitive microphone can be divided into two portions. The first portion varies with acoustic signal and is desirable. The second portion, or parasitic capacitance portion, does not vary with acoustic signal. The second portion is related to the construction of the microphone and is undesirable as it degrades performance. This parasitic capacitance portion should be minimized. Berggvist et al. attaches the two electrodes together at the end of the arms (26). Although the area is small, the parasitic capacitance is relatively large.
It is the object of the present invention to overcome the disadvantages of the prior art by at least achieving a high sensitivity with a small diaphragm, reducing the die size, and reducing the parasitic capacitance. Other features and advantages will be apparent to those skilled in the art with reference to the below description and the Figures.